The invention relates to a method of producing an integrated optical waveguide circuit made up of a glass substrate which is provided with grooves. Glass, having a refractive index which is higher than the refractive index of the substrate, is deposited on the substrate in such a manner that the grooves are filled with the deposited glass. Thereafter, the excess deposited glass is removed, to the extent that the deposited glass remains only in the grooves.
Published Japanese Patent Application (Kokai) No. 53-70939 describes a method of producing an optical waveguide circuit. According to an English-language abstract published in Patents Abstracts of Japan (Volume 2, page 5556E78, 1978), the following procedure is followed. Grooves, which can be smoothed by means of etching or fire polishing if desired, are mechanically formed in a glass, for example quartz, base plate. A mixture of gases such as SiCl.sub.4, GeCl.sub.4 and O.sub.2 is supplied to the base plate, and by means of a CVD method a fine powder of SiO.sub.2 and GeO.sub.2 is deposited on the base plate. The deposited powder is then vitrified by heating. Thereafter, the excess deposited glass is removed, by polishing, to the extent that deposited glass remains only in the grooves. The glass remaining in the grooves forms a low-loss optical waveguide. If so desired, a protective layer of SiO.sub.2 and B.sub.2 O.sub.3 may be provided on the glass plate. The Figures reproduced in the abstract show that the grooves in the base plate have a rectangular cross-section.
The above-described method can be effectively performed with only a limited choice of starting materials because in this method, both during deposition of the fine SiO.sub.2 and GeO.sub.2 powder and during the subsequent vitrification, the substrate must be heated to rather high temperatures. The substrate and the grooves provided therein must retain their shapes at these high temperatures. As a result, in the majority of cases the use of quartz substrates is required.
A further limitation of the described method is that the coefficients of thermal expansion of the deposited glass and the substrate material must not differ greatly. Otherwise during cooling, after vitrification, the deposited glass may become loosened from the substrate.
Finally, the rectangular cross-section of the grooves in the above-described optical waveguide circuits is optically disadvantageous, as will become apparent below.